Superconductivity refers to those properties of a material which enable it to conduct electrical current with zero resistance. This effect was discovered in 1911 by Onnes [Comm. Phys. Lab. Univ. Leiden Nos. 119, 120, 122 (1911)] and studied extensively by researchers in various laboratories.
In most of the materials studied to date the superconducting properties vanish at temperatures larger than a critical temperature T.sub.c which is determined primarily by empirical methods. Thus, elements, such as Pb and Nb, have been found to be superconducting at very low temperatures. Metals like Nb are presently used in the construction of high field magnets and other applications where electrical energy dissipation is a major factor. The technological utility of such metals is limited due to the expensive refrigeration systems required to cool the materials to liquid helium temperatures of about 4.2.degree. K in order for the superconducting properties to be present.
The search for superconductors with higher transition temperatures has led to the discovery of various compounds with T.sub.c .congruent. 20.degree. K. Some examples are Nb.sub.3 Ge, Nb.sub.3 Sn, PdH, PdD, and PdCuH. The highest T.sub.c material discovered so far is Nb.sub.3 Ge with T.sub.c = 23.2.degree. K. As such, costly refrigeration methods must also be employed to use these compounds as superconducting wires. Also, many of the presently available high temperature superconductors such as Nb.sub.3 Ge with T.sub.c = 23.degree. K, are difficult to manufacture and contain relatively expensive ingredients.
It should be noted that some substances are not superconducting at all. This group of materials includes the elements Cu, Ni, Pd and compounds such as NiH.
The large scale applications of superconductivity include energy transmission, energy generation, magnetic levitation of trains, and many other uses with involve electrical current flow. At present, these applications are limited primarily by the high cost of refrigeration required to cool the materials to their superconducting temperatures, and by the capital costs involved in the development and production of appropriate superconducting materials. It has not been possible, heretofore, to obtain superconductivity at temperatures above T.sub.c = 25.degree. K or, more importantly, at about the boiling point of liquid nitrogen, i.e., 77.degree. K.